1. Field
The present invention relates to a nonaqueous-electrolyte battery and a battery pack.
2. Description of the Related Art
Nonaqueous-electrolyte batteries which are charged/discharged based on the movement of lithium ions between the negative electrode and positive electrode are being extensively investigated and developed as batteries having a high energy density.
These nonaqueous-electrolyte batteries are desired to have various properties according to applications. For example, the batteries used as the power sources of digital cameras are expected to be discharged at about 3 C, while the batteries mounted on vehicles such as, e.g., hybrid electric cars are expected to be discharged at about 10 C or higher. Because of this, nonaqueous-electrolyte batteries for use in such applications are especially desired to have high-current characteristics and an excellent charge/discharge cycle life in repetitions of charge/discharge at a high current.
At present, nonaqueous-electrolyte batteries employing a lithium-transition metal composite oxide as a positive active material and a carbonaceous substance as a negative active material have been commercialized. In the lithium-transition metal composite oxide, the transition metal generally is cobalt, manganese, nickel, or the like.
In recent years, a nonaqueous-electrolyte battery employing as a negative active material a lithium-titanium composite oxide having a higher lithium insertion/release potential than carbonaceous substances was proposed. Lithium-titanium composite oxides change little in volume with charge/discharge and hence have excellent cycle characteristics. Of these composite oxides, spinel lithium titanate is especially promising.
For example, JP-A 8-22841 discloses a battery employing a spinel lithium-manganese oxide in the positive electrode and a spinel lithium-titanium oxide in the negative electrode. This battery constitution enables the nonaqueous-electrolyte secondary battery to withstand severe charge/discharge.
As a result of investigations diligently made by the present inventors, it has been found that the batteries employing a lithium-titanium composite oxide as a negative active material suffer a large amount of self-discharge during storage.